U.S. patent application number 10/169226 was filed with the patent office on 2003-11-13 for grain boundary materials as electrodes for lithium ion cells.
Invention is credited to Beaulieu, Luc Y.J., Dahn, Jeffrey R., Fredericksen, Brian D., Larcher, Dominique C..
Application Number | 20030211390 10/169226 |
Document ID | / |
Family ID | 29400702 |
Filed Date | 2003-11-13 |
United States Patent
Application |
20030211390 |
Kind Code |
A1 |
Dahn, Jeffrey R. ; et
al. |
November 13, 2003 |
Grain boundary materials as electrodes for lithium ion cells
Abstract
An electrode composition for a lithium ion battery comprising
particles having a single chemical composition. The particles
consist of (a) at least one metal element selected from the group
consisting of tin, aluminum, silicon, antimony, lead, germanium,
magnesium, zinc, cadmium, bismuth, and indium; (b) at least one
metal element selected from the group consisting of manganese,
molybdenum, niobium, tungsten, tantalum, iron, copper, titanium,
vanadium, chromium, nickel, cobalt, zirconium, tantalum, scandium,
yttrium, ruthenium, platinum, and rhenium; and, optionally, (c)
carbon, and have a microstructure characterized by a plurality of
electrochemically inactive, nanometer-sized crystalline grains
separated by electrochemically active non-crystalline regions.
Inventors: |
Dahn, Jeffrey R.; (Nova
Scotia, CA) ; Beaulieu, Luc Y.J.; (Nova Scotia,
CA) ; Larcher, Dominique C.; (Cedex, FR) ;
Fredericksen, Brian D.; (Watertown, MN) |
Correspondence
Address: |
Lucy C Weiss
Office of Intellectual Property Counsel
3M Innovative Properties Company
PO Box 33427
St Paul
MN
55133-3427
US
|
Family ID: |
29400702 |
Appl. No.: |
10/169226 |
Filed: |
November 13, 2002 |
PCT Filed: |
December 22, 2000 |
PCT NO: |
PCT/US00/35331 |
Current U.S.
Class: |
429/218.1 ;
429/220; 429/221; 429/224; 429/229; 429/231.5; 429/231.6;
429/231.8; 429/231.95 |
Current CPC
Class: |
H01M 4/587 20130101;
H01M 4/58 20130101; Y02E 60/10 20130101; H01M 4/38 20130101; H01M
10/0525 20130101; H01M 2004/021 20130101; H01M 4/134 20130101; H01M
4/364 20130101; H01M 4/02 20130101 |
Class at
Publication: |
429/218.1 ;
429/229; 429/231.6; 429/224; 429/231.5; 429/221; 429/220;
429/231.8; 429/231.95 |
International
Class: |
H01M 004/38; H01M
004/46; H01M 004/42; H01M 004/58 |
Claims
What is claimed is:
1. An electrode composition for a lithium ion battery comprising
particles having a single chemical composition, said particles
consisting of (a) at least one metal element selected from the
group consisting of tin, aluminum, silicon, antimony, lead,
germanium, magnesium, zinc, cadmium, bismuth, and indium; (b) at
least one metal element selected from the group consisting of
manganese, molybdenum, niobium, tungsten, tantalum, iron, copper,
titanium, vanadium, chromium, nickel, cobalt, zirconium, tantalum,
scandium, yttrium, ruthenium, platinum, and rhenium; and,
optionally, (c) carbon, said particles having a microstructure
characterized by a plurality of electrochemically inactive,
nanometer-sized crystalline grains separated by electrochemically
active non-crystalline regions.
2. An electrode composition according to claim 1 wherein said
particles consist of (a) tin; (b) at least one metal element
selected from the group consisting of manganese, molybdenum,
niobium, tungsten, tantalum, iron, copper, titanium, vanadium,
chromium, nickel, cobalt, zirconium, tantalum, scandium, yttrium,
ruthenium, platinum, and rhenium; and, optionally, (c) carbon.
3. An electrode composition according to claim 1 wherein said
particles consist of (a) at least one metal element selected from
the group consisting of tin, aluminum, silicon, antimony, lead,
germanium, magnesium, zinc, cadmium, bismuth, and indium; (b) iron;
and, optionally, (c) carbon.
4. An electrode composition according to claim 1 wherein said
particles consist of (a) at least one metal element selected from
the group consisting of tin, aluminum, silicon, antimony, lead,
germanium, magnesium, zinc, cadmium, bismuth, and indium; (b)
manganese; and, optionally, (c) carbon.
5. An electrode composition according to claim 1 wherein said
particles consist of tin, manganese, and carbon in the form of
SnMn.sub.3C.
6. An electrode composition according to claim 1 wherein said
particles consist of tin, iron, and carbon in the form of
SnFe.sub.3C.
7. An electrode composition according to claim 1 wherein said
particles range in size from about 2 microns to about 30
microns.
8. An electrode composition according to claim 1 wherein said
crystalline grains are no greater than about 20 nanometers.
9. An electrode composition according to claim 1 wherein said
non-crystalline regions represent at least 10% by volume of said
particle calculated from transmission electron microscopy assuming
spherical grains.
10. A lithium ion battery comprising: (a) a first electrode
comprising particles having a single chemical composition, said
particles consisting of (i) at least one metal element selected
from the group consisting of tin, aluminum, silicon, antimony,
lead, germanium, magnesium, zinc, cadmium, bismuth, and indium;
(ii) at least one metal element selected from the group consisting
of manganese, molybdenum, niobium, tungsten, tantalum, iron,
copper, titanium, vanadium, chromium, nickel, cobalt, zirconium,
tantalum, scandium, yttrium, ruthenium, platinum, and rhenium; and,
optionally, (iii) carbon, said particles having a microstructure
characterized by a plurality of electrochemically inactive,
nanometer-sized crystalline grains separated by electrochemically
active non-crystalline regions; (b) a counterelectrode; and (c) an
electrolyte separating said electrode and said
counterelectrode.
11. A battery according to claim 10 wherein said particles consist
of (a) tin; (b) at least one metal element selected from the group
consisting of manganese, molybdenum, niobium, tungsten, tantalum,
iron, copper, titanium, vanadium, chromium, nickel, cobalt,
zirconium, tantalum, scandium, yttrium, ruthenium, platinum, and
rhenium; and, optionally, (c) carbon.
12. A battery according to claim 10 wherein said particles consist
of (a) at least one metal element selected from the group
consisting of tin, aluminum, silicon, antimony, lead, germanium,
magnesium, zinc, cadmium, bismuth, and indium; (b) iron; and,
optionally, (c) carbon.
13. A battery according to claim 10 wherein said particles consist
of (a) at least one metal element selected from the group
consisting of tin, aluminum, silicon, antimony, lead, germanium,
magnesium, zinc, cadmium, bismuth, and indium; (b) manganese; and,
optionally, (c) carbon.
14. A battery according to claim 10 wherein said particles consist
of tin, manganese, and carbon in the form of SnMn.sub.3C.
15. A battery according to claim 10 wherein said particles consist
of tin, iron, and carbon in the form of SnFe.sub.3C.
16. A battery according to claim 10 wherein said particles range in
size from about 2 microns to about 30 microns.
17. A battery according to claim 10 wherein said crystalline grains
are no greater than about 20 nanometers.
18. A battery according to claim 10 wherein said non-crystalline
regions represent at least 10% by volume of said particle.
Description
STATEMENT OF PRIORITY
[0001] This application derives priority from a provisional
application filed Dec. 28, 1999, entitled "Grain Boundary Materials
as Anodes for Lithium Ion Cells" bearing serial No. 60/173364, the
contents of which are hereby incorporated by reference.
TECHNICAL FIELD
[0002] This invention relates to anode compositions useful in
lithium ion cells.
BACKGROUND
[0003] Two classes of materials have been proposed as anodes for
lithium ion cells. One class includes materials such as graphite
and carbon that are capable of intercalating lithium. While the
intercalation anodes generally exhibit good cycle life and
coulombic efficiency, their capacity is relatively low. In
particular, graphite can intercalate lithium to a maximum of 1
lithium atom per six carbon atoms. This corresponds to a specific
capacity of 373 mAh/g of carbon. Because the density of graphite is
2.2 g/cc, this translates to a volumetric capacity of 818 mAh/cc.
Other types of carbon have higher specific capacity values, but
suffer from one or more disadvantages such as relatively low
density, unattractive voltage profiles, and large irreversible
capacity that limit their utility in commercial lithium ion
cells.
[0004] A second class includes metals that alloy with lithium
metal. These alloy-type anodes generally exhibit higher capacities
relative to intercalation-type anodes. For example, specific
capacity associated with the formation of a lithium-aluminum alloy
is 992 mAh/g. The corresponding value for the formation of a
lithium-tin alloy is 991 mAh/g. One problem with such alloys,
however, is that they can exhibit relatively poor cycle life and
coulombic efficiency due to fragmentation of the alloy particles
during the expansion and contraction associated with compositional
changes in the alloy.
SUMMARY
[0005] The invention provides electrode compositions suitable for
use in lithium ion batteries in which the electrode compositions
have high initial capacities that are retained even after repeated
cycling. The electrode compositions, and batteries incorporating
these compositions, are also readily manufactured.
[0006] To achieve these objectives, the invention features, in a
first aspect, an electrode composition that includes particles
having a single chemical composition formed from (a) at least one
metal element selected from the group consisting of tin, aluminum,
silicon, antimony, lead, germanium, magnesium, zinc, cadmium,
bismuth, and indium; (b) at least one metal element selected from
the group consisting of manganese, molybdenum, niobium, tungsten,
tantalum, iron, copper, titanium, vanadium, chromium, nickel,
cobalt, zirconium, tantalum, scandium, yttrium, ruthenium,
platinum, and rhenium; and, optionally, (c) carbon. The particles
have a microstructure characterized by a plurality of
electrochemically inactive, nanometer-sized crystalline grains
separated by electrochemically active non-crystalline regions.
[0007] As used herein, a "particle" is a component of a powder.
Each particle is made up of many crystalline "grains." A
crystalline grain is a region of the particle from which
diffraction occurs coherently (i.e., the crystal axes have fixed
directions within the grain). The crystalline grains are separated
by non-crystalline regions. These regions are characterized by a
lower degree of order compared to the crystalline grains.
[0008] A "single chemical composition" means that when the sample
is analyzed by transmission electron microscopy, the types of atoms
that are detected are the same, on a nanometer scale range,
regardless of where the electron beam is placed within the
sample.
[0009] An "electrochemically active" material is a material that
reacts with lithium under conditions typically encountered during
charging and discharging in a lithium battery.
[0010] An "electrochemically inactive" material is a material that
does not react with lithium under conditions typically encountered
during charging and discharging in a lithium battery.
[0011] Examples of useful particles include those characterized by
the chemical composition SnMn.sub.3C and SnFe.sub.3C. These
materials have electrochemically inactive crystalline grains, yet
form useful electrode materials owing to the presence of
electrochemically active tin atoms in the non-crystalline regions
separating the crystalline grains. Preferably, the particles have a
size ranging from about 2 microns to about 30 microns (measured by
scanning electron microscopy). The crystalline grains preferably
are no greater than about 20 nanometers where this figure refers to
the length of the longest dimension of the grain. The
non-crystalline regions preferably from at least about 10% by
volume of the particle, calculated from transmission electron
microscopy data assuming spherical grains.
[0012] The details of one or more embodiments of the invention are
set forth in the accompanying drawings and the description below.
Other features, objects, and advantages of the invention will be
apparent from the description and drawings, and from the
claims.
DESCRIPTION OF DRAWINGS
[0013] FIG. 1 is an x-ray diffraction profile for a SnMn.sub.3C
sample prepared by ball milling for 20 hours. All observed
diffraction peaks are from SnMn.sub.3C.
[0014] FIG. 2 is an x-ray diffraction profile for a SnFe.sub.3C
sample prepared by ball milling for 20 hours. All observed
diffraction peaks are from SnFe.sub.3C.
[0015] FIG. 3 illustrates the cycling performance, in terms of
voltage versus capacity and capacity versus cycle number, for two
Li/SnMn.sub.3C cells.
[0016] FIG. 4 illustrates the cycling performance, in terms of
differential capacity versus voltage, for a Li/SnMn.sub.3C
cell.
[0017] FIG. 5 is a series of x-ray diffraction profiles for a
Li/SnMn.sub.3C cell obtained during discharge.
[0018] FIG. 6 is a series of Mossbauer spectroscopy scans for a
Li/SnMn.sub.3C cell obtained during discharge.
[0019] FIG. 7 illustrates the variation of the Mossbauer center
shift of the minority component of a Li/SnMn.sub.3C cell during
charge and discharge.
[0020] FIG. 8 is a series of x-ray diffraction profiles for both an
unheated SnMn.sub.3C sample and for samples heated to 400.degree.
C., 500.degree. C., and 600.degree. C.
[0021] FIG. 9 illustrates the cycling performance, in terms of
voltage versus capacity and capacity versus cycle number, for cells
constructed using the samples described in FIG. 8.
[0022] FIG. 10 includes a series of x-ray diffraction profiles for
both an unheated SnFe.sub.3C sample and for samples heated to
100.degree. C., 200.degree. C., and 300.degree. C., and further
illustrates the cycling performance, in terms of voltage versus
capacity, for cells constructed using these materials.
[0023] FIG. 11 includes a series of x-ray diffraction profiles for
both an unheated SnFe.sub.3C sample and for samples heated to
400.degree. C., 500.degree. C., and 600.degree. C., and further
illustrates the cycling performance, in terms of voltage versus
capacity, for cells constructed using these materials.
[0024] FIGS. 12 and 13 are transmission electron micrographs of a
SnMn.sub.3C sample.
DETAILED DESCRIPTION
[0025] The electrode compositions are in the form of powders made
up of particles. The particles have the chemical composition and
microstructure described in the Summary of the Invention, above.
The powders may be prepared directly using techniques such as
ball-milling. Alternatively, the powders may be prepared in the
form of thin films using techniques such as sputtering, chemical
vapor deposition, vacuum deposition, vacuum evaporation, melt
spinning, splat cooling, spray atomization, and the like, and then
pulverized to form powders.
[0026] The electrode compositions are particularly useful as anodes
for lithium ion batteries. To prepare a battery, the electrode
powder is combined with a binder (e.g., a polyvinylidene fluoride
binder) and solvent to form a slurry which is then coated onto a
backing using conventional coating techniques and dried to form the
anode. The anode is then combined with an electrolyte and a cathode
(the counterelectrode). The electrolyte may be a solid or liquid
electrolyte. Examples of solid electrolytes include polymeric
electrolytes such as polyethylene oxide, polytetrafluoroethylene,
fluorine-containing copolymers, and combinations thereof. Examples
of liquid electrolytes include ethylene carbonate, diethyl
carbonate, propylene carbonate, and combinations thereof. The
electrolyte is provided with a lithium electrolyte salt. Examples
of suitable salts include LiPF.sub.6, LiBF.sub.4, and
LiClO.sub.4.
[0027] Examples of suitable cathode compositions for liquid
electrolyte-containing batteries include LiCoO.sub.2,
LiCo.sub.0.2NiO.sub.2, and Li.sub.1.07Mn.sub.1.93O.sub.4. Examples
of suitable cathode compositions for solid electrolyte-containing
batteries include LiV.sub.3O.sub.8 and LiV.sub.2O.sub.5.
[0028] The invention will now be described further by way of the
following examples.
EXAMPLES
[0029] Ball Milling Procedure
[0030] A Spex 8000 high-impact mixer mill was used to violently
shake sealed, hardened steel vials for periods up to about 40
hours. In an argon-filled glove box, the desired amounts of
elemental powders or intermetallic phases were added to the vial,
along with several hardened steel balls measuring 12.7 mm in
diameter. The vial was then sealed and transferred to the mill
where it was shaken violently. The milling time was selected to be
sufficient to reach milling equilibrium. In general, milling times
were on the order of about 16 hours.
[0031] Cycling Behavior
[0032] Electrodes were prepared by coating slurries of the powders
onto a copper foil and then evaporating the carrier solvent. In a
typical preparation, about 82% by weight powder (prepared by ball
milling), 10% by weight Super S carbon black (MMM carbon, Belgium),
and 8% by weight polyvinylidene fluoride (Atochem) were thoroughly
mixed with N-methyl pyrrolidinone by stirring in a sealed bottle to
make a slurry; the polyvinylidene fluoride was pre-dissolved in the
N-methyl pyrrolidinone prior to addition of the powder and carbon
black. The slurry was spread in a thin layer (about 150 micrometers
thick) on the copper foil with a doctor-blade spreader. The sample
was then placed in a muffle oven maintained at 105.degree. C. to
evaporate the N-methyl pyrrolidinone over a 3 hour period.
[0033] Circular electrodes measuring 1 cm in diameter were cut from
the dried film using an electrode punch. The electrodes were
weighed, after which the weight of the copper was subtracted and
the active mass of the electrode calculated (i.e., the total weight
of the electrode multiplied by the fraction of the electrode made
of the active electrode powder). The circular electrodes were then
heat-sealed in polyethylene bags until further use.
[0034] The electrodes were used to prepare coin cells for testing.
All cell construction and sealing was done in an argon-filled glove
box. A lithium foil having a thickness of 125 micrometers
functioned as the anode and reference electrode. The cell featured
2325 hardware, equipped with a spacer plate (304 stainless steel),
and a disc spring (mild steel). The disc spring was selected so
that a pressure of about 15 bar would be applied to each of the
cell electrodes when the cell was crimped closed. The separator was
a Celgard #2502 microporous polypropylene film (Hoechst-Celanese)
that had been wetted with a 1 M solution of LiPF.sub.6 dissolved in
a 30:70 volume mixture of ethylene carbonate and diethyl carbonate
(Mitsubishi Chemical).
[0035] After construction, the cells were removed from the glove
box and cycle tested using a MACCOR constant current cycler.
Cycling conditions were typically set at a constant current of 37
mA/g of active material. Cutoff voltages of 0.0 V and 1.3 V were
used.
[0036] X-Ray Diffraction
[0037] Powder x-ray diffraction patterns were collected using a
Siemens D5000 diffractometer equipped with a copper target x-ray
tube and a diffracted beam monochromator. Data was collected
between scattering angles of 10 degrees and 80 degrees unless
otherwise noted.
[0038] To examine the electrode materials during cycling, in-situ
x-ray diffraction experiments were performed. Cells for in-situ
x-ray diffraction were assembled as described above in the case of
the cycling experiment with the following differences. The coin
cell can was provided with a circular hole measuring 18 mm in
diameter. A 21 mm diameter beryllium window (thickness=250
micrometers) was affixed to the inside of the hole using a pressure
sensitive adhesive (Roscobond from Rosco of Port Chester, N.Y.).
The electrode material was coated directly onto the window before
it was attached to the can.
[0039] The cell was mounted in a Siemens D5000 diffractometer and
slowly discharged and charged while x-ray diffraction scans were
taken continuously. Typically, a complete scan took 2-5 hours and
the discharge and charge time took 40-60 hours, giving
approximately 10-30 "snapshots" of the crystal structure of the
electrode as a function of its state of charge. The voltage of the
cell was continuously monitored during cycling.
[0040] Mossbauer Spectroscopy
[0041] In-situ .sup.119 mSn Mossbauer spectroscopy was used to
study the local environment of tin atoms during reaction with
lithium. The advantage of Mossbauer spectroscopy is that it can
distinguish between tin atoms within the non-crystalline regions
and tin atoms within the crystalline grains.
[0042] Room temperature Mossbauer measurements were made with a
Wissel System II constant acceleration spectrometer operating at a
frequency of 23 Hz and a krypton/CO.sub.2 x-ray proportional
counter (Reuter-Stokes Inc.). The detector employed a Pd filter.
Data were collected using an Ortec ACE multi-channel scaling board.
The Ca.sup.119 mSnO.sub.3 source had an intrinsic line width of
0.78 mm/s (FWHM), and the velocity scale was calibrated using a
mixed sample of tin and BaSnO.sub.3. Elevated temperature
measurements were made using a small heater placed around the
sample without blocking the gamma rays.
[0043] Powder samples were prepared as follows. Powders were
manually ground and sieved (-325 mesh). Typically, 150 mg of powder
was uniformly distributed over a 30 mm piece of Scotch Brand
adhesive tape (3M Co., St. Paul, Minn.), and was kept in place by
another piece of tape on top. Total measurement times ranged
between 3 and 24 hours.
[0044] The cell used for in-situ Mossbauer measurements was similar
to the cell used for in-situ x-ray spectroscopy except that it was
designed for maximum transmission of gamma rays. As such, all steel
parts were removed (including the spacer and spring), and a second
hole (diameter=13 mm) was cut in the cell top. A second piece of
beryllium (diameter=15 mm, thickness=1 mm) was placed over the hold
and held in place by Roscobond pressure sensitive adhesive. A thin
bead of Torr Seal (high vacuum grade available from Varian) was
applied following cell assembly at the interface between the cell
bottom and beryllium piece, and at the interface between the cell
top and beryllium piece. Electrodes, prepared as described above,
were coated directly onto the beryllium.
[0045] The cell was held in place approximately 10 cm from the
detector and 1 cm from the source. Charging and discharging
currents were controlled by a Keithley 220 programmable current
source interfaced to a computer equipped with a general purpose
interface bus. Voltages were measured using a Keithley 196 digital
voltmeter. Spectra were obtained continuously while the cell was
discharged and subsequently charged. The total experiment time was
approximately 180 hours, during which about 60 three-hour Mossbauer
spectra were recorded. The spectra were fitted with one or more
Lorentzian-shaped peaks. The center shift, area, and half-width of
the fitted peaks were monitored.
[0046] Transmission Electron Microscopy
[0047] Samples were prepared for transmission electron microscopy
by dispersing the powder in methanol and sonicating the dispersion
for one minute. Next, one drop of the sonicated dispersion was
placed on a standard 3 mm transmission electron microscopy grid
(carbon/formvar thin film supported on a copper mesh grid). Excess
solution was wicked away with a wedge of filter paper and the
remaining sample was allowed to dry for 10 minutes before inserting
it into the microscope.
[0048] Transmission electron microscopy and electron diffraction
analysis were performed on a Hitachi H9000 instrument operating at
300 kV. Energy dispersive x-ray spectroscopy was performed on the
same instrument using a Noran Voyager X-Ray Spectroscopy
System.
[0049] Specific samples were prepared and tested as follows.
Example 1
[0050] An intermetallic compound, SnMn.sub.3C, was prepared by
adding stoichiometric ratios of 0.800 g tin powder (Aldrich
Chemical), 1.111 g manganese powder (Aldrich Chemical), and 0.081 g
graphite powder (mesocarbon microbeads from Osaka Gas Ltd. that had
been heated to 2650.degree. C.), along with two 12.7 mm diameter
hardened steel balls, to a hardened steel vial in an argon-filled
glove box. The vial was placed in the Spex 8000 mixer and subjected
to maximum milling intensity for 20 hours following the general
procedure described above.
[0051] The x-ray diffraction pattern of the milled sample is shown
in FIG. 1. It agrees with the literature pattern for SnMn.sub.3C
except that the Bragg peaks are broad (width=about 1 degree),
indicating the presence of nanometer-sized grains. Using the
Scherer formula, L=0.9.lambda./(Bcos.theta.), where L is the grain
size, .lambda. is the x-ray wavelength (1.54178 .ANG.), B is the
full width at half maximum of a particular x-ray peak in radians,
and .theta. is the Bragg angle of the peak, the grain size is
calculated to be about 8 nanometers. The particle size of the
sample was in the range of 2-50 micrometers, determined by scanning
electron microscopy, demonstrating that each particle was made up
of many grains.
[0052] An electrochemical cell was constructed as described above
and its cycling behavior tested. FIG. 3a shows the voltage-capacity
for the cell. The cell exhibited a reversible capacity of about 130
mAh/g.
[0053] FIG. 3b shows the capacity versus cycle number for the cell
depicted in FIG. 3a, and for an identical cell. Both show no loss
in capacity over 100 cycles. One of the cells was slowed to 18.5
mA/g at cycle 120, and to 9 mA/g at cycle 160. At the lowest
current, a capacity of 150 mAh/g was observed. This corresponds to
a volumetric capacity of about 1200 mAh/g (calculated based upon a
density value of 7.9 g/cc for SnMn.sub.3C.
[0054] FIG. 4 shows the differential capacity versus voltage at
several cycle numbers for the cell that was slowed. The
differential capacity shows a stable pattern over the first 150
cycles, characteristic of nanometer-sized tin grains in a matrix.
No sharp peaks in differential capacity develop, indicating that
there is no aggregation of tin into large regions and that the tin
atoms are active. If all the tin atoms were active, and each could
react with 4.4 Li/Sn, then the specific capacity of SnMn.sub.3C
would be about 400 mAh/g. The observed value of 150 mAh/g
corresponds to about 1.5 Li/Sn.
[0055] In-situ x-ray diffraction measurements were made using a
specific current of 2.2 mA/g. X-ray scans of 3 hours duration were
taken successively. FIGS. 5(a)-(d) show the x-ray diffraction
pattern from the electrode during discharge; FIG. 5(e) shows
voltage versus capacity (bottom axis) and versus scan number (top
axis) for the sample. Each diffraction pattern represents the sum
of five adjacent x-ray scans to improve the signal to noise ratio.
The x-ray data demonstrate that even though approximately 2 Li/Sn
have reacted with the electrode (calculated coulombmetrically based
on the current, electrode mass, and time of current flow), there is
no change in the position or intensity of the main Bragg peaks
attributed to SnMn.sub.3C at 32, 39, and 40.degree.. On the other
hand, the broad "hump" near 22.degree. intensifies as the discharge
process proceeds.
[0056] The fact that the Bragg peaks do not change is evidence that
the nanocrystalline grains do not react with lithium at all.
Accordingly, the only materials available to react with lithium are
the tin atoms located in non-crystalline regions separating the
grains. The intensification of the "hump" near 22.degree. may be
the result of small amounts (e.g., on the order of a few atoms) of
Li.sub.4Sn in the non-crystalline regions.
[0057] In-situ Mossbauer spectroscopy measurements were made using
a discharge current of 2.2 mA/g following the procedure described
above. Spectra of 3 hours duration were collected continuously.
FIGS. 6(a), (b), and (c) show the first, twentieth, and fortieth
scans. FIG. 6(d) shows voltage versus capacity (bottom axis) and
versus scan number (top axis) for the sample. The first spectrum
(FIG. 6(a)) was fitted with a major component with a center shift
near 1.7 mm/s and a minor component with a center shift near 2.5
mm/s. A third component with a center shift near 0.0 mm/s was also
included, but it was not needed in order to obtain a good fit.
Because x-ray diffraction data showed that the nanometer-sized
crystalline grains did not react with lithium, the center shift and
half-width of the major component were kept fixed while fitting the
spectra taken as the discharge proceeded.
[0058] FIGS. 6(b) and (c) show that the minor component shifts to
smaller velocity as lithium reacts with the sample. The Mossbauer
spectra demonstrate that the average center shift changes from
about 2.5 to about 1.8 as lithium reacts with tin. Accordingly, the
shift of the minor component is consistent with the reaction of
lithium with tin.
[0059] FIG. 7 shows the variation of the center shift of the minor
component as a function of scan number taken during discharge and
charge. The current used during charge was 3.3 mA/g. The change in
the center shift is reversible. This is evidence for the reversible
reaction of lithium with tin atoms located within the
non-crystalline regions of the sample.
[0060] FIGS. 12 and 13 are transmission electron micrographs taken
of the sample at both high (400,000.times.) and low (20,000.times.)
magnification. The micrographs show the presence of two types of
particles. The first type ranges in size from 10 nm to over 10
microns. These particles are composed of crystalline grains having
a size in the 8 nanometer range. The grains are separated from each
other by non-crystalline regions that are significantly less
ordered than the crystalline grains. The scanned area exhibited a
single diffraction pattern The second type of particle is a single
crystal roughly on the order of 10-30 nanometers by 100-300
nanometers with a large aspect ratio (somewhere between 10:1 and
20:1).
Example 2
[0061] Three additional samples of SnMn.sub.3C were prepared
following the procedure of Example 1. The samples were heat-treated
at 400.degree. C., 500.degree. C., and 600.degree. C.,
respectively, under vacuum for 3 hours. The x-ray diffraction
spectra for the three samples, as well as the sample from Example 1
prepared without heat-treating, are shown in FIG. 8. As shown in
FIG. 8, the widths of the Bragg peaks of the SnMn.sub.3C phase
narrow as the temperature increases, consistent with a growth of
the size of the nanometer-sized crystalline grains and a reduction
in the number of atoms in the non-crystalline regions. FIG. 8 also
shows evidence of some minor impurities, representing Fe--C phases,
formed during heating as a result of iron contamination during
milling.
[0062] FIG. 9 shows the voltage versus capacity and capacity versus
cycle number results for cells made from these samples. The cells
containing heat-treated material show much smaller capacity
compared to the cell containing unheat-treated material, of which
about 15 mAh/g originates from the Super S carbon black used to
prepare the electrode composition. These results are further
evidence that heat treatment induces grain growth, thereby
decreasing the size of the non-crystalline regions and reducing the
reversible capacity of the materials. The reduction in capacity, in
turn, is related to a decrease in the number of tin atoms in the
non-crystalline regions available for reaction with lithium.
Example 3
[0063] The procedure of Example 1 was followed except that 0.823 g
tin powder, 1.160 g iron powder (Aldrich Chemical Co.), and 0.084 g
graphite powder were used to prepare a material having the formula
SnFe.sub.3C. The x-ray diffraction pattern of the material is shown
in FIG. 2. It agrees with the literature pattern for SnFe.sub.3C
except that the Bragg peaks are broad, indicating the presence of
nanometer-sized grains. The particle size of the sample was in the
range of 2-50 micrometers, determined using scanning electron
microscopy, demonstrating that each particle was made up of many
grains.
Example 4
[0064] Six additional samples of SnFe.sub.3C were prepared
following the procedure of Example 2. The samples were heat-treated
at 100.degree. C., 200.degree. C., 300.degree. C., 400.degree. C.,
500.degree. C., and 600.degree. C., respectively, under vacuum for
3 hours. The x-ray diffraction spectra for these six samples, as
well as the sample from Example 2 prepared without heat-treating,
are shown in FIG. 10. As shown in FIG. 10, the widths of the Bragg
peaks of the SnFe.sub.3C phase narrow as the temperature increases,
consistent with a growth of the size of the crystalline grains and
a reduction in the number of atoms in the non-crystalline
regions.
[0065] FIG. 11 shows the voltage versus capacity and capacity
versus cycle number results for cells made from these samples. The
cells containing heat-treated material show much smaller capacity
compared to the cell containing unheat-treated material, of which
about 15 mAh/g originates from the Super S carbon black used to
prepare the electrode composition. These results are further
evidence that heat treatment induces grain growth, thereby
decreasing the width of the non-crystalline regions and reducing
the reversible capacity of the materials. The reduction in
capacity, in turn, is related to a decrease in the number of tin
atoms in the non-crystalline regions available for reaction with
lithium.
[0066] A number of embodiments of the invention have been
described. Nevertheless, it will be understood that various
modifications may be made without departing from the spirit and
scope of the invention. Accordingly, other embodiments are within
the scope of the following claims.
* * * * *